Xenotransplantation - Nuffield Council on Bioethics

Animal-to-Human Transplants : the ethics of xenotransplantation
2.21 Infection is a major problem with artificial lungs, since they are in constant contact
with the atmosphere and do not have the natural protection mechanisms found in
human lungs. Blood clotting is another problem that must be overcome before
artificial lung implants become a realistic option.
Bioengineered organ and tissue replacements
2.22 As indicated above, reproducing the complicated functions of the body in a purely
mechanical way is extremely difficult. This has led to the development of artificial
organs incorporating living cells, tissues or organs which can perform some of the
functions of the tissue or organ that needs replacing.
Liver
2.23 The liver performs a wide range of important functions including the processing of
the products of digestion, the control of the metabolism of proteins and
carbohydrates, the manufacture of essential proteins such as blood clotting factors,
the removal of toxic substances absorbed from the gut, the excretion of the
breakdown products of red blood cells and the secretion of bile. It is impossible at
present to begin to replicate all of these complicated functions by wholly artificial
means, so research has focused instead upon bioengineered livers. The intention is
to use them on a short-term basis to keep patients suffering from liver failure alive
while they wait for a transplant or to give their own liver a chance to regenerate.
At the simplest level, the patient’s blood is passed through an animal liver: at best
this can keep patients alive for a very short time. 23
2.24 Implantable artificial livers are a long way off, but clinical trials are under way for
artificial livers which function outside the body. One such device is the
extracorporeal liver assist device, which has been used successfully in small numbers
of patients. 24 The device filters blood through thousands of tiny porous tubes.
Human liver cells are packed around the tubes to process the passing blood and to
get rid of waste. The liver cells are immortalised (they have the ability to keep
reproducing) and can be grown in the laboratory. It is important that the liver cells
do not enter the blood, as there is a chance that if they passed into the body and
became established in an organ or tissue, they might develop into tumours.
23
Chari R et al. (1994) Brief report: Treatment of hepatic failure with ex vivo pig-liver perfusion followed by liver
transplantation. New England Journal of Medicine, 331:234-7.
24
McNair A N B et al. (1995) Recent advances : hepatology. British Medical Journal, 311:1351-5.
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